Micron and Microscopica Ada, Vol. 23. No. 3, pp. 337—339, 1992. 0739—6260/92$5.00 +0.00Printed in Great Britain. 1992PergamonPressLtd

SHORTCOMMUNICATION

AN UNUSUAL TYPE OF CORNEAL NIPPLE IN THEEARWIG, FORFICULA SP.,WITH A POSSIBLE

ANTI-REFLECTION ROLE

SUDIP DEY andBEGONIA DKHAR

ElectronMicroscopeLaboratory,RegionalSophisticatedInstrumentionCentre,North-EasternHill University,Shillong 793 003, India

(Received5 September1991; revised 16 January 1992)

Abstract—Anunusualtypeof cornealnipple pattern,with featuresdifferent from the usuallower height (GroupI) andgreaterheight (GroupII andIII) nipples,hasbeendetectedin thecornealsurfaceof theearwig,Forficula sp. However,thenipplesshowsomesimilaritieswitheachof thetwo abovetypespossessingcertaincharacters.The heightofthenipples,theirshapeandarrangementin thefacetsuggestthatthenipplesmayactasananti-reflectiondevice,andthusincreasetheintensityof transmittedlight, therebyaiding scotopicvision oftheinsect.

INTRODUCTIONThecompoundeyeof insectsconsistsof astructuralandfunctionalunit, known as

an ommatidium. Eachommatidiumhas a cornea!lens, a crystalline coneand therhabdomor photoreceptor.The cornea!lens is a modified cuticle which is castoffduring eachact of ecdysis.In additionto its normal functionas a refractivestructure,the lens-cuticleperformsextensiveandvariedopticalfunctions,suchasaninterferencefilter, selectiveabsorptionof light, enhancedreflectionat selectivewavelengthsandasan anti-reflectiondevice (Bernhardet a!., 1970;Miller, 1979;Miller et a!., 1966;Dey,1988). As an adaptationto the light environment,the lens-cuticleof certain insectsshows ultrastructural specialization,such as the presenceof conical cuticularprotuberances,termedthe cornealnipple. The reducedreflection observedin thecorneaof somenocturnalmothswasattributedtotheseprotuberancesandit is arguedthatscotopicvision maybe aidedby thepresenceof thesestructuresby increasingtheintensity of transmittedlight (Bernhardet a!., 1970). However, the greaterheightcornealnipples(morethan50 nmhigh) whichcanactasananti-reflectiondevicehavebeenreported only in insectsbelongingto advancedorders.Studieson structuralspecializationin relationto photic adaptationin the lowergroupsof insectshavenotappearedin the literature.

Scanningelectronmicroscopy,due to its largedepth of field and high resolvingpower,has beenused to reveal the surfaceultrastructuralfeaturesof many oculartissuesand could provide much information, therebyexplaining many of the lessunderstoodproblemsassociatedwith vision.

Thepresentstudywasthereforeaimedat determiningwhetherthereisa relationshipbetweenthe ultrastructuralfeatureof a singlefacetof theearwig(Dermaptera)anditsscotopicvision.

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338 S. Dey andB. Dkhar

MATERIALS AND METHODSTheeyeswerefixedin 3% glutaraldehydein 0.1 M sodiumcacodylatebufferat4~C

for 4 h. The sampleswere washedwith the buffer, dehydratedthrough increasingconcentrationsof acetone,then critical point dried in a Samdri PVT 3 (Tousimis)critical point dryerusingacetoneas intermediateandcarbondioxide as transitionalfluid. The dry sampleswere then coatedwith gold in a JFC 1100 (Jeol) ion sputtercoater and were observedin a JSM 35CF (Jeol) scanningelectron microscopeoperatingat an acceleratingvoltageof 15 kY.

RESULTS AND DISCUSSIONThescanningelectronmicrographof the cornealsurfaceof Forficula sp.,shown in

Fig. la, revealedcertaininterestingfeatures.Comparisonof the surfacepatternof asinglefacetwith that of the housefly,Muscadomestica(Fig. ib), andnocturnalmoth,Attacusatlas (Fig. lc), showedthat the cornealprotuberancesof Forficula sp. havea

(ci) —, .~.

7

(b) (c)

Fig. I. Scanningelectronmicrographof a ~ingIefacet showingthecornealntpple patternofForficula sp., as comparedto that of Musa domesticaand Attacus atlas. (a) Forficula sp.

(bar= I Mm); (b) Musca domestica(bar— I Mm): (c) Attacusat!a.s (bar=0.1 Mm).

CornealNipple in the Earwig 339

numberof similaritiesas well asdifferencesfrom thoseof thenocturnalmothandthehousefly. As farasthe height (distancebetweenbaseandtip) is concerned,the nipplesat mostof thefacetpartsaremoreor lesssimilarto thoseof thenocturnalmoth,buttheperiodicity (centreto centredistance)is not constant.Further,in somepartsof thefacet,thenipplesareflat andaresimilarto thoseof thehousefly;whereasin someparts,thenipplesappeartoassumeaconicalshapewith considerableheightwith referencetothe cuticularsurface(70—140nm) andare moreor lesssimilar to thatof the nocturnalmoth (180 nm).

Thecornealnipplesare classifiedon the basisof their height asGroupI (50 nmorless), GroupII (between50 nm and 200 nm) and GroupIII (200nm or greater)protuberances(Bernhardeta!., 1965).AlthoughGroupI protuberancesweredetectedin thefacetsurfaceof manyinsectspeciesbelongingto differentorders,theirfunctioniscompletelyunknown.GroupII andIII nipples,on the otherhand,play adefinite anti-reflection role due to their conical shape,regularperiodicity and height. However,reportson theoccurrenceof thesegreaterheightnipplesin insectsotherthannocturnalLepidopterais very rare (Dey, 1988).Thepioneeringwork of Bernhardet a!. (1970)showedthatnippleswith greaterheighttend to occurin insectsof anadvancedorder,andlowerheight nipplesaremostlyfoundin insectsrepresentinglessadvancedordersindicatingthe absenceof any anti-reflectiondeviceassociatedwith them.Ourpresentobservationson the cornealsurfaceof theearwig,however,showan exception.In themajorportionof thefacet,it canbeconsideredthat thecorneawith a refractiveindexn = 1.5 is coveredwith athin film (cornealnipple)of refractiveindexn2 andthicknessd.The effectiveindex of refractionof the nipple increasesgraduallyfrom the refractiveindex of air at the tip to that of the corneaat thebase.It is well known that if n2 is agradual transition betweenn1 and n3 with a thicknessd=A0/2, a broad bandanti-reflectioneffect is achieved(Miller, 1979).Sincein the presentcase,mostof thenipples fulfil this physical condition, the same anti-reflection effect is expected.However, it may not be as efficient as in nocturnalmoths,as it appearsthat theperiodicityof thenipplesis notregular.Nevertheless,thenipplesare likely to help thescotopicvisionof the insectby increasingtheintensityof transmittedlight. Thepresentstudyfurther suggeststhat anti-reflectiondevicesin the facetsurfaceneednot be acharacteristicfeatureofhighergroupsalone,andmaybeexpectedin lowergroups,alsoasan adaptationto scotopicvision.

Acknowledgements—Theauthorsare thankful to Prof. D. T. Khathing,Head, R.S.I.C. for constantencouragement,to Mr. A. Shadapfor photography,alsoto Mrs. RajammaJohnsfor typingthemanuscript.

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